1. No category

Analysis of Vinegar by
Titration
Lab 10
Outline
• Purpose
• Potentiometric Titrations
•
•
•
•
•
– Proticity
– Equivalence
– pKa
Vinegar Titration
Calculations
Waste
Following Lab Reminder
pH Probe Calibration
Purpose
Students will use a pH probe to perform
potentiometric titrations to determine the
mass percent of acetic acid in vinegar and
the pKa of acetic acid.
Potentiometric Titrations
Potentiometric titrations can be used to
determine:
• proticity (how many acidic hydrogens are
donated to solution)
• the amount or concentration of acid or base
present (using MaVa= MbVb)
• pKa (the “– log” of the acid dissociation
constant)
Proticity
Acetic acid is a monoprotic weak acid that reacts with
NaOH or KOH in a 1:1 ratio and produces a single
sigmoidal curve.
12.00
11.00
10.00
9.00
8.00
pH 7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
Volume, mL
20.00
25.00
Equivalence
20
18
16
14
12
10
8
6
4
2
0
0.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
3.00
5.00
10.00
15.00
Volume, mL
20.00
25.00
pH
Derivative
The equivalence point volume is determined by plotting
a derivative curve of the titration curve. The steepest
point on the derivative curve corresponds to the
equivalence point volume. (Find the exact value on
your spreadsheet!)
Equivalence Point Volume
• The concentration of acetic acid can
be determined from:
– the equivalence point volume of the base
– the concentration of the base
– the volume of the acid used in the
titration
• The equation to use is:
– MaVa = MbVb
pKa
The pKa of acetic acid is the pH at the halfequivalence point volume of the titration,
because:
For a weak acid:





H
A
HA
+
and Ka =    
HA 
At the half-equivalence point, half the acid
has been converted to its salt, so:
[HA] = [A-]
Ka = [H+]
pKa = pH
H+
A-
Vinegar Titration
• Make the required dilution of vinegar.
• Calibrate your pH probe.
• Titrate the specified aliquots to obtain
titration curves.
• Determine the volume of base delivered at
each time point, using the base delivery
rate.
• Graph the derivative of your titration
curves following the instructions in the
manual.
• Make up a spreadsheet that will allow you
to calculate the indicated values.
Spreadsheet
Time, s
0.01
35.95
60.24
79.47
103.54
142.46
151.31
187.66
222.13
268.71
pH
Vol, mL
3.02
0.02
4.15
1.97
4.65
5.21
4.74
10.44
5.16
13.45
5.34
16.11
8.50
20.89
10.58
21.52
11.35
22.95
11.73
25.36
Deriv
0.001
0.000
0.003
0.000
0.002
0.010
18.750
1.960
0.001
0.000
pKa
Half-equivalence
Point Volume
Highest
derivative
Equivalence
Point Volume
Calculations
• At the equivalence point:
moles of base = moles of acid
(because they react in a 1:1 ratio)
• Moles of base = (Eq Pt Vol base, L) x ([base], M)
• Mass of acetic acid = (molar mass of acetic acid, g/mol)
x (moles of acid, moles)
• Mass of vinegar = (density, g/mL) x (Vol vinegar, mL)
• Mass % = (mass of acetic acid / mass of vinegar) x
100%
Safety Concerns
• Reagents:
 Acetic Acid (1 N)
 Sodium Hydroxide (0.1 N) / Potassium Hydroxide (0.1 N)
• Eye Contact:
 Irritation, tearing, redness, pain, impaired vision, severe
burns and irreversible eye injury.
• Skin Contact:
 Severe skin irritation, soreness, redness, destruction of
skin (penetrating ulcers) . May cause sensitization and / or
allergic reaction.
• Inhalation:
 May cause coughing, serious burns, pneumonitis,
pulmonary edema, and coma.
• Ingestion:
 Toxic. Corrosive to mucous membranes. May cause
perforation of the esophagus and stomach, abdominal pain,
nausea, vomiting, diarrhea, general gastro-intestinal upset.
Waste
• Only solutions with a pH between 6 and
8 can go down the drain.
• All other solutions need to go in the
acid/waste container in the fume hood.
Lab 11 Reminder
• Lab 11 is next.
pH Probe Calibration
• Calibrate your pH probe according to the
instructions in your lab manual.
• Be careful not to break the bulb.
• Be careful not to contaminate the buffer
solutions.
• Ask your instructor if you need help.